ABSTRACT. This article provides a critical overview of competing theories of conceptual structure (definitional structure, probabilistic structure, theory structure), including the view that concepts have no structure (atomism). We argue that the explanatory demands that these different theories answer to are best accommodated by an organization in which concepts are taken to have atomic cores that are linked to differing types of conceptual structure. Margolis, E., & Laurence, S. (2003). Concepts. In S. Stich & T. Warfield (eds.), The Blackwell Guide to the Philosophy of Mind (Blackwell), pp. 190-213. DOI: 10.1002/9780470998762.ch8